Tools can now live-preview fx while color dialogs are active

This is a pretty sweet improvement, and one I didn't think would work
(due to the way VB handles modal dialogs).

In a nutshell, the color selection user control can now raise Change
events *while a modal color selection dialog is active*!  This allows
the user to see the effects of varying colors inside the current tool
window, without having to close the color dialog.
Updated split-toning Balance formula

Per discussion on 6114f65b501dae19acc04c424608414e7929952c, this update
includes a modified Split Tone dialog UI (with color selectors), and an
improved Balance formula.  I think it's working properly now... (fingers
crossed).

I may still add a Strength slider to the tool, as that would allow for
partial split-toning (as opposed to overwriting the image's current
colors, as it does now).  But that will be handled in a separate commit
at some later time.